Engineers Explain Why the Twin Towers Collapsed (2023)

In the years since the terrorist attacks in New York City, individual engineers and committees of experts have studied the crumpling of the World Trade Center twin towers. By examining the building's destruction step-by-step, experts are learning how buildings fail and discovering ways to build stronger structures by answering the question: What caused the twin towers to fall?

Aircraft Impact

When hijacked commercial jets piloted by terrorists struck the twin towers, some 10,000 gallons (38 kiloliters) of jet fuel-fed an enormous fireball. But the impact of the Boeing 767-200ER series aircraft and the burst of flames did not make the towers collapse right away. Like most buildings, the twin towers had a redundant design, which means that when one system fails, another carries the load.

Each of the twin towers had 244 columns around a central core that housed the elevators, stairwells, mechanical systems, and utilities. In this tubular design system, when some columns became damaged, others could still support the building.

"Following the impact, floor loads originally supported by the exterior columns in compression were successfully transferred to other load paths," wrote examiners for the official Federal Emergency Management Agency (FEMA) report. "Most of the load supported by the failed columns is believed to have transferred to adjacent perimeter columns through Vierendeel behavior of the exterior wall frame."

Belgian civil engineer Arthur Vierendeel (1852-1940) is known for inventing a vertical rectangular metal framework that shifts shear differently than diagonal triangular methods.

(Video) This Computer Simulation Explains How the Twin Towers Fell

The impact of the aircraft and other flying objects:

  1. Compromised the insulation that protected the steel from high heat
  2. Damaged the sprinkler system of the building
  3. Sliced and cut many of the interior columns and damaged others
  4. Shifted and redistributed the building load among columns that were not immediately damaged

The shift put some of the columns under "elevated states of stress."

Heat From Fires

Even if the sprinklers had been working, they could not have maintained enough pressure to stop the fire. Fed by the spray of jet fuel, the heat became intense. It is no comfort to realize that each aircraft carried less than half of its full capacity of 23,980U.S. gallons of fuel.

Jet fuel burns at 800 to 1,500 degrees Fahrenheit. This temperature is not hot enough to melt structural steel. But engineers say that for the World Trade Center towers to collapse, its steel frames didn't need to melt—they just had to lose some of their structural strength from the intense heat. Steel will lose about half its strength at 1,200 Fahrenheit. Steel also becomes distorted and will buckle when the heat is not a uniform temperature. The exterior temperature was much cooler than the burning jet fuel inside. Videos of both buildings showed inward bowing of perimeter columns resulting from sagging of heated trusses on many floors.

(Video) Engineers' Stories: Since 9/11

Collapsing Floors

Most fires start in one area and then spread. Because the aircraft hit the buildings at an angle, the fires from impact covered several floors almost instantly. As the weakened floors began to bow and then collapse, they pancaked. This means that upper floors crashed down on lower floors with increasing weight and momentum, crushing each successive floor below.

"Once movement began, the entire portion of the building above the area of impact fell in a unit, pushing a cushion of air below it," wrote researchers of the official FEMA report. "As this cushion of air pushed through the impact area, the fires were fed by new oxygen and pushed outward, creating the illusion of a secondary explosion."

With the weight of the plunging floors' building force, the exterior walls buckled. Researchers estimate that the "air ejected from the building by gravitational collapse must have attained, near the ground, the speed of almost 500 mph." Loud booms were heard during the collapse. They were caused by airspeed fluctuations reaching the speed of sound.

Why They Flattened

Before the terrorist attack, the twin towers were 110 stories tall. Constructed of lightweight steel around a central core, the World Trade Center towers were about 95 percent air. After they collapsed, the hollow core was gone. The remaining rubble was only a few stories high.

Engineers Explain Why the Twin Towers Collapsed (1)

(Video) How the Twin Towers Fell | The World Trade Center and How It Collapsed

Strong Enough?

The twin towers were built between 1966 and 1973. No building constructed at that time would have been able to withstand the impact of the terrorist attacks in 2001. We can, however, learn from the collapse of the skyscrapers and take steps to construct safer buildings and minimize the number of casualties in future disasters.

When the twin towers were constructed, the builders were granted some exemptions from New York's building codes. The exemptions allowed the builders to use lightweight materials so the skyscrapers could achieve great heights. According to Charles Harris, author of "Engineering Ethics: Concepts and Cases," fewer people would have died on 9/11 if the twin towers had used the type of fireproofing required by older building codes.

Others say the architectural design actually saved lives. These skyscrapers were designed with redundancies—anticipating that a small plane could accidentally penetrate the skyscraper skin and the building would not fall from that type of accident.

(Video) NIST's World Trade Center Investigations

Both buildings withstood the immediate impact of the two large aircraft bound for the West Coast on 9/11. The north tower was hit at 8:46 a.m. ET, between floors 94 and 98—it did not collapse until 10:29 a.m., which gave most people one hour and 43 minutes to evacuate. Even the south tower was able to stand for a remarkable 56 minutes after being hit at 9:03 a.m. ET. The second jet hit the south tower on lower floors, between floors 78 and 84, which structurally compromised the skyscraper earlier than the north tower. Most of the south tower occupants, however, began evacuating when the north tower was hit.

The towers could not have been designed any better or stronger. Nobody anticipated the deliberate actions of an aircraft filled with thousands of gallons of jet fuel.

9/11 Truth Movement

Conspiracy theories often accompany horrific and tragic events. Some occurrences in life are so shockingly incomprehensible that some people begin to doubt theories. They might reinterpret evidence and offer explanations based on their prior knowledge. Passionate people fabricate what becomes alternative logical reasoning. The clearinghouse for 9/11 conspiracies became The mission of the 9/11 Truth Movement is to reveal what it believes to be the United States' covert involvement in the attacks.

When the buildings collapsed, some thought it had all of the characteristics of a "controlled demolition." The scene in Lower Manhattan on 9/11 was nightmarish, and in the chaos, people drew on past experiences to determine what was happening. Some people believe that the twin towers were brought down by explosives, although others find no evidence for this belief. Writing in the Journal of Engineering Mechanics ASCE, researchers have shown "the allegations of controlled demolition to be absurd" and that the towers "failed due to gravity-driven progressive collapse triggered by the effects of fire."

(Video) NIST Video: Why the Building (WTC7) Fell

Engineers examine evidence and create conclusions based on observations. On the other hand, the Movement seeks the "suppressed realities of September 11th" that will support their mission. Conspiracy theories tend to continue in spite of the evidence.

Legacy on Building

While architects strive to design safe buildings, developers don't always want to pay for over-redundancies to mitigate outcomes of events that are unlikely to happen. The legacy of 9/11 is that new construction in the United States must now adhere to more demanding building codes. Tall office buildings are required to have more durable fireproofing, extra emergency exits, and many other fire safety features. The events of 9/11 changed the way we build, at local, state, and international levels.

Additional Sources


What was the reason of twin tower fall? ›

The collapse of the World Trade Center occurred on Tuesday, September 11, 2001, after two commercial airliners hijacked by Al-Qaeda were deliberately flown into the Twin Towers of the World Trade Center complex in New York City as part of the September 11 attacks.

How long did it take to clean up 9 11? ›

The process of cleanup and recovery at Ground Zero took a total of eight months, with rescue and recovery workers working 24 hours a day. The remains of all the other buildings in the World Trade Center were demolished. And finally, by the end of May, the last piece of steel was ceremonially removed.

How hot did the Twin Towers get? ›

NIST reported maximum upper layer air temperatures of about 1,000 degrees Celsius (1,800 degrees Fahrenheit) in the WTC towers (for example, see NCSTAR 1, Figure 6-36).

Did anyone survive September 11? ›

Four people who were in the concourse area between the Twin Towers survived and either saved themselves or were rescued. Nobody who was in the South Tower at the time of its collapse survived. The last survivor removed from the WTC collapse debris was found in the ruins of the North Tower 27 hours after its collapse.

How far did the dust cloud travel on 9 11? ›

Researchers specializing in aerosol science were able to determine that the plume of particulate matter and noxious gases created by the collapsed buildings shot up nearly a mile into the sky. It then spread out 44 miles above all of Manhattan and drifted across the East River into Brooklyn.

How long did it take for the South Tower to collapse? ›

9:58:59: The South Tower of the World Trade Center collapses, 56 minutes after the impact of Flight 175. Impact speed is considered one of the likely factors during its much faster collapse unlike that of the North Tower.

Did any firefighters survive 911? ›

On the day following the attacks, eleven people were rescued from the rubble, including six firefighters and three police officers. One woman was rescued from the rubble, near where a West Side Highway pedestrian bridge had been. Two PAPD officers, John McLoughlin and Will Jimeno, were also rescued.

Why did they not rebuild the Twin Towers? ›

Due to the completion of One, Three, and Four World Trade Center, as well as the memorial and museum, the project was very unlikely to be built.

How many people are still missing from 9 11? ›

The last two recent identifications were made last year. There are 1,106 victims who remain unidentified.

How deep in the ground was the Twin Towers? ›

The World Trade Center was built entirely on landfill. As a result, the site's foundations, including those of the Twin Towers, had to extend down to bedrock about 70 feet below street level. During construction, workers removed more than one million cubic yards of soil and rock from the site.

Did anyone get to the roof of the Twin Towers? ›

Though some people were rescued from the World Trade Center's roof after the 1993 basement garage bombing, fire evacuation plans for the buildings never included a rooftop escape.

Why were the Twin Towers built so high? ›

Designed to be very tall to maximize the area of the plaza, the towers were initially to rise to only 80-90 stories. Only later was it decided to construct them as the world's tallest buildings, following a suggestion said to have originated with the Port Authority's public relations staff.

What floor did the plane hit? ›

What happened at the World Trade Center on 9/11? The hijacked Flight 11 was crashed into floors 93 to 99 of the North Tower (1 WTC) at 8:46 a.m. The hijacked Flight 175 struck floors 77 to 85 of the South Tower (2 WTC) 17 minutes later at 9:03 a.m.

Who was pulled from the rubble of 9 11? ›

Guzman-McMillan was the last survivor to be pulled from the 9/11 wreckage. She will recount her courageous story about surviving the fall of the Twin Tower and her wait for rescue in the rubble.

How many firefighters died on 9 11? ›

On Sept. 11, 2001, 343 firefighters died, lost in the collapse of buildings, from smoke inhalation and from more of the devastating impacts that day.

How long did it take for planes to fly again after 9 11? ›

All air service in the United States was suspended on 9/11, but the aviation system was restored within days. Passenger travel by commercial airlines did not recover until March 2004 when the number of passengers enplaned returned to the August 2001 level.

How long after 9 11 did planes not fly? ›

In the immediate aftermath of the attacks, North American airspace was closed to civilian traffic for two days, but flights slowly resumed after.

How long did it take to land all planes on 9 11? ›

Pick an airport.” Sliney says 700 of the 4,000 planes in the air landed within 10 minutes. 10 a.m. UAL 93 passengers vote to retake the aircraft.

How many children died in 911? ›

At two and a half years old, Christine Lee Hanson was the youngest of the eight children who were killed on 9/11, all passengers aboard the aircraft commandeered by terrorists.

What tower was hit first? ›

The first impact was that of American Airlines Flight 11, which crashed into the North Tower of the World Trade Center complex in Lower Manhattan at 08:46. Seventeen minutes later, at 09:03, the World Trade Center's South Tower was hit by United Airlines Flight 175.

Did the South Tower hit first or second? ›

Thirty-four minutes later, at 9:37 a.m., American Airlines Flight 77, crashes into the Pentagon collapsing part of the building. Then at 9:59 a.m., the burning WTC South Tower collapses. Four minutes later, at 10:03 a.m., United Airlines Flight 93, crashes in a field near Shanksville, Pennsylvania.

What was found under the twin towers? ›

In 2010, workers excavating the World Trade Center site discovered a 30-foot long section of the wooden vessel 20-30 feet below street level. A year later, they uncovered a three-foot section of the ship's bow.

Who was the highest ranking firefighter killed on 9 11? ›

CHIEF: The Story of Bill Feehan, The Oldest and Highest Ranking Firefighter Killed on 9/11. This heartbreaking film tells the story of the beloved FDNY Fire Chief who died at his command station on 9/11 when the south tower of the World Trade Center collapsed.

Who was the oldest firefighter killed on 911? ›

When the south tower toppled and fell onto his command post, William M. Feehan perished. At age 71 he was the oldest first responder killed on 9/11.

What is replacing the Twin Towers? ›

One World Trade Center (One WTC), also called Freedom Tower, skyscraper in New York, New York, that is the centrepiece of reconstruction at Ground Zero, the site of the former World Trade Center complex.

Were there apartments in the Twin Towers? ›

And in fact, by 1979, almost a decade after its first tenants moved in, there was still more than a million square feet of unoccupied office space at the complex. That's on top of the huge chunk of the Twin Towers that had no private tenants.

Who owns the World Trade Center? ›

The Port Authority of New York and New Jersey owns and operates the Oculus/Transportation Hub, Liberty Park, and other public open space and infrastructure on site.

What happened to the bodies on Flight 11? ›

Many of the bodies of the passengers aboard the two airplanes that struck the buildings were consumed by burning jet fuel, leaving only traces of DNA, much of it so damaged that it was impossible to read. Few bodies were found intact.

How many missing persons are never found in the world? ›

According to the NamUs database, there are 600,000 people declared missing every year. Alongside that statistic, there are 4,400 unidentified bodies discovered every year. That means only 0.7333% of people who go missing are found and unable to be identified.

What happened to the bodies in the World Trade Centre? ›

We believe in the free flow of information

Amid the destruction, an improvised team of volunteers, firefighters, police and detection dogs found 21 people alive on the first day, but none thereafter. The remaining body parts would be painstakingly collected in 21,900 pieces scattered throughout the skyscrapers' debris.

Who is the owner of Twin tower? ›

RK Arora is a real estate tycoon in Haryana, Delhi, Uttar Pradesh, and other parts of North India. He is the owner and managing director of the construction company Supertech.

Were the Twin Towers built on landfill? ›

Aside from withstanding enormous wind loads, the World Trade Center towers were also constructed to withstand settlement loads. Because the towers were built on six acres of landfill, the foundation of each tower had to extend more than 70 feet below ground level to rest on solid bedrock.

How deep is the fountain at the 9 11 Memorial? ›

The pits, 192 feet by 192 feet and 30 feet deep, are lined in black granite—black as death. Water cascades down the four walls and disappears into a square hole in the center of the pool. The effect is quite beautiful—and the sound of the cataracts effectively masks the noise of the surrounding city.

How long was the elevator ride to the top of the Twin Towers? ›

In just 47 seconds, visitors to the top of 1 World Trade Center will witness 515 years of New York City history.

Did anyone survive the twin tower crash? ›

Though they didn't know it at the time, Mr Clark and Mr Praimnath had escaped just four minutes before the collapse. Of all the people in the floors above where Flight 175 crashed, only four survived. Mr Clark and Mr Praimnath are two of them. Looking back years later, Mr Clark still can't explain why he's alive.

Did the elevators work on 9 11? ›

In a fire emergency, an elevator is programmed to return to its lowest floor and hold its doors open. On Sept. 11, many elevators far below the crash zones failed to do this, although they continued to have electrical power.

Is one World Trade Center bigger than the Twin Towers? ›

Form and facade. The building occupies a 200-foot (61 m) square, with an area of 40,000 square feet (3,700 m2), nearly identical to the footprints of the original Twin Towers. The tower is built upon a 185-foot (56 m) tall windowless concrete base, designed to protect it from truck bombs and other ground-level attacks.

What metal was the Twin Towers made of? ›

The major structural material employed in the towers was A36 structural steel, although higher strength steel was used in the lower elevations of the structure.

Was the Twin Towers bigger than the Empire State Building? ›

Each of the center's twin towers will be eight stories and 100 feet—taller than the Empire State Building.

What was the worst plane crash on ground? ›

The most fatalities in any aviation accident in history occurred during 1977 in the Tenerife airport disaster, when 583 people were killed when two Boeing 747s collided on a runway.

How long did Ground Zero burn? ›

The work was so dangerous that many firefighters and police officers wrote their names and phone numbers on their forearms in case they fell into the hole or were crushed. Did you know? Fires continued to burn in lower Manhattan for 99 days after the attack.

Did 747 have an upstairs? ›

The Boeing 747's iconic upper deck became part of its design when engineers considered how they could streamline the aircraft's cargo operations. Moving the cockpit upstairs allowed a cargo door to be incorporated into the aircraft's nose, meaning that operators could load freight into the plane more efficiently.

Who were the 500,000 rescued by on 9 11? ›

500,000 people were transported to safety in approximately nine hours by hundreds of vessels that answered a call from the U.S. Coast Guard to converge on New York Harbor to aid in the evacuation. This extraordinary rescue was memorialized in the 2011 short documentary film Boatlift.

How long did it take to clean up the Twin Towers? ›

The process of cleanup and recovery at Ground Zero took a total of eight months, with rescue and recovery workers working 24 hours a day. The remains of all the other buildings in the World Trade Center were demolished. And finally, by the end of May, the last piece of steel was ceremonially removed.

How much did it cost to clean up 9 11? ›

The final cost of the cleanup and restoration of the World Trade Center site is expected to be about $1.5 billion. Another $16.4 billion will be required to replace or repair destroyed and damaged buildings at the site and its adjacent areas and to replace the buildings' contents.

How many paramedics died in 911? ›

Eight EMTs and paramedics were killed on 9/11. Read about the eight EMS providers here. Since September 11, 2001, hundreds of public safety personnel have died from occupational illness related to their rescue and recovery work.

How many floors did firefighters climb on 911? ›

– On this Sunday, 21 years ago, first responders climbed 110 stories at the World Trade Center after the twin towers were attacked. A total of 343 firefighters lost their lives in the line of duty on that horrific day.

What are the names of the 343 firefighters that died on 9 11? ›

Their names were firefighters Gerard Baptiste, Vernon Cherry, Tarel Coleman, Andre Fletcher, Keith Glascoe, Ronnie Henderson, William Henry, Karl Joseph, Keithroy Maynard, Vernon Richard, Shawn Powell, and Leon Smith Jr.

Why did the North Tower last longer? ›

The difference in how long the towers remained standing - the south tower for 56 minutes and 10 seconds, the north for 102 minutes and 5 seconds - may also be due to the greater speed of the plane that hit the south tower and the fact that it hit 10 floors lower.

Which plane hit the towers first? ›

The first impact was that of American Airlines Flight 11, which crashed into the North Tower of the World Trade Center complex in Lower Manhattan at 08:46. Seventeen minutes later, at 09:03, the World Trade Center's South Tower was hit by United Airlines Flight 175.

Did they rebuild the Twin Towers? ›

The World Trade Center (WTC) is a mostly completed complex of buildings in the Lower Manhattan neighborhood of New York City, U.S., replacing the original seven buildings on the same site that were destroyed in the September 11 attacks.

How many planes were hijacked and crashed on 9 11? ›

On the morning of 11 September 2001, 19 Al Qaeda terrorists hijacked four commercial passenger planes in the United States. Two planes were flown into the Twin Towers of the World Trade Center in New York City, causing both towers to collapse. A third plane was crashed into the Pentagon, just outside Washington, DC.

Did the Twin Towers sway in the wind? ›

Each of the Twin Towers had 110 floors. Each tower's footprint and floors were approximately an acre in size. On windy days, each tower could sway up to almost 12 inches side to side. There were 43,600 windows in the Twin Towers, equating to more than 600,000 square feet of glass.

Did anyone survive the North Tower? ›

Clark was one of only 18 people in the South Tower to escape from within or above the impact zone where the plane struck, escaping from his office on floor 84. No one escaped at or above the impact point in the North Tower.

What tower is replacing the Twin Towers? ›

One World Trade Center (One WTC), also called Freedom Tower, skyscraper in New York, New York, that is the centrepiece of reconstruction at Ground Zero, the site of the former World Trade Center complex.

How many people died in 911? ›

Where did the plane land after 911? ›

During the flight, she heard the pilot announce a delay in landing, but did not realize how serious the situation was until her plane landed a thousand miles away at Gander International Airport in Newfoundland, Canada.

Who is the owner of Twin Towers? ›

World Trade Center (1973–2001)
World Trade Center
Opening1 WTC: December 15, 1970 2 WTC: January 1972 3 WTC: July 1, 1981 4 WTC: January 1977 5 WTC: March 1972 6 WTC: January 1974 7 WTC: May 1987
InauguratedApril 4, 1973
DestroyedSeptember 11, 2001
OwnerPort Authority of New York and New Jersey
23 more rows

How did 911 hijackers get into cockpit? ›

Reports from flight attendants

Ong said she thought that the hijackers had "jammed their way" in. The commission suggested they attacked the flight attendants to get a cockpit key, to force one of them to open the cockpit door, or to lure the captain or first officer out of the cockpit.

Who was the youngest person on Flight 93? ›

Deora Frances Bodley of San Diego, California was the youngest person aboard Flight 93.

How many survivors were rescued on 9 11? ›

Their rescue was later portrayed in the Oliver Stone film, World Trade Center. In total, twenty survivors were pulled out of the rubble. The final survivor, Port Authority secretary Genelle Guzman-McMillan, was rescued 27 hours after the collapse of the North Tower.


1. How 9/11 Changed Skyscraper Design - Cheddar Explains
2. How Did Jet Fuel Melt The Steel Beams In The Twin Towers? | Blowing-Up History
(Discovery UK)
3. World Trade Center and how it collapsed
4. Forensic Engineering World Trade Center 7 | CSI | Forensics Talks Ep12 | ft. Dr. Leroy Hulsey
(3D Forensics)
5. 9/11 Impact Anatomy | National Geographic
(National Geographic)
6. Inside Twin Tower's Structure and the 911 Attack - Original World Trade Center
(3D Living Studio)


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